The invention is directed to a method to inhibit the growth of cancer cells by contacting the cells (either in vitro or in vivo) with a gadolinium-containing agent to increase specifically the neutron capture cross-section within the nuclei of the cancerous cells. Upon exposure to a neutron flux, the cancer cells containing the gadolinium-containing agents are specifically destroyed or their growth is inhibited.
Complete bibliographic citations to the references discussed herein are contained in the Bibliography section, immediately preceding the Claims.
Modern medical research often benefits from collaborations between physicists and medical doctors. Magnetic resonance imaging, computerized axial tomography, and laser surgery are only a few offshoots of such teamwork, demonstrating that an interdisciplinary view of medicine has become essential. Now-rampant medical conditions such as cardiovascular disease, dementia, and cancer, are actively being explored by a diversity of scientists. In this same vein of interdisciplinary investigation, the present application discloses a new therapy for treating cancers (i.e., neoplastic growths) in general, and, in the preferred embodiment of the invention, malignant brain cancers.
Neutron Capture Therapy (NCT) is a non-invasive, experimental therapy that has been proposed in the past to treat malignant gliomas. The conventional therapy is based on a binary approach: In the first step, the patient is intravenously injected with an NCT-enhancing agent; that is, a tumor-seeking compound containing an isotope that has a capture cross section for thermal neutrons many times greater than any other elements present in the surrounding tissue. In the second step, the patient""s skull is exposed to thermal neutrons. A portion of the thermal neutrons are captured by the NCT-enhancing agent, thereby inducing in the NCT agent a localized, biologically-destructive nuclear reaction.
An NCT agent that has been extensively investigated is the isotope boron-10 (10B), which undergoes the reaction 10B(n,xcex1)7Li. When 10B is present in tissue irradiated by a neutron flux, almost all of the radiation dose delivered to the individual cells results from the high linear energy transfer (xe2x80x9cLET,xe2x80x9d i.e., stopping power) fission products of the boron neutron capture reaction. For many years, the greatest challenge for NCT using boron-containing sensitizers has been to link the 10B isotope to a tumor-seeking compound, thereby adding greater specificity to the radiation dose. In short, absent specificity for tumor cells, NCT is no more efficacious than conventional radio-therapies, i.e, non-specific radiation treatments that deliver the radiation dose equally to malignant and non-malignant cells alike. The key to unlocking the potential of NCT is to deliver a radiotoxic dose only to tumor cells, thereby sparing the surrounding healthy tissue from radiation-induced damage.
The discovery of two boronated compounds that exhibited tumor-selective uptake has resulted in clinical trials of Boron Neutron Capture Therapy (BNCT). These trials are currently underway in Europe, Japan, and the United States.(1)(2)(3) The uptake mechanisms of these compounds into tumor cells are thought to be different, and a major effort is being put into the synthesis of novel compounds for BNCT utilizing alternative cancer targeting strategies.(4)(5)(6) 
To date, however, the full potential of NCT as a means to treat cancers remains largely unrealized. There remains a long-felt and unsatisfied need for biologically well-tolerated compounds that simultaneously exhibit very high neutron capture cross-sections, high levels of tumor-specific uptake, and high levels of tumor cell-specific kill or growth inhibition. The present invention addresses this long-felt need.
Compounds containing gadolinium 155 (155Gd) and/or gadolinium-157 (157Gd) are a powerful alternative NCT isotope as compared to 10B. Gd neutron capture therapy, designated herein as xe2x80x9cGdNCT,xe2x80x9d has never been clinically tested.(7)(8)(9) The present inventors have determined, however that certain Gd-containing compounds are excellent neutron capture agents for several reasons, including the following:
157Gd, which is found with a natural abundance of 15.7%, is the most effective isotope in terms of neutron capture cross-section, having the largest thermal neutron cross-section (254,000 barn) of all stable isotopes currently known. For sake of comparison, 10B has a neutron capture cross-section of 3,840 barn, 16O=0.00019 barn, 12C=0.0035 barn, 1H=0.333 barn, and 14N=1.83 barn.
Some gadolinium compounds are known to accumulate in brain tumors and not in the surrounding healthy tissue. Some of these compounds currently are used as tumor contrast-enhancing agents for magnetic resonance imaging (MRI) due to the large magnetic moment of the Gd3+ ion.(11) 
While the Gd3+ ion is itself toxic, its usefulness in MRI stimulated the search for compounds such as the Gd-DTPA complex described herein, which is both stable in the blood stream and non-toxic. The pharmacokinetics, biodistribution and tolerance of Gd-DTPA and other Gd-containing compounds used as MRI contrast agents are well documented.(12)(13) 
The gadolinium neutron capture reaction, 157Gd(n,xcex3)158Gd, provokes complicated nuclear decay transitions that generate prompt xcex3 emission up to 7.8 MeV, accompanied by the emission of internal conversion electrons, mostly Auger electrons in the energy range of about 41 keV and below. Both xcex3 rays and Auger electrons are low LET radiation, with contrasting ranges and biological effects in tissue. Gamma rays travel through the whole thickness of the tissue, and are weakly absorbed by both healthy and tumor tissues. Hence these capture products would deliver dose widely, independent of the precise location of a GdNCT agent in the tumor cells.
By contrast, Auger electrons, which are mass and charge-carrying, are highly ionizing over a short range. The longest radiation length is on the order of tens of nanometers for the most energetic electrons. Most favorably for GdNCT, Auger electrons appear to induce double-stranded DNA damage when the Gd is sufficiently close to the DNA.(14) Consequently, the dose enhancement due to the electrons emitted in the GdNCT reaction would be most substantial when the electrons originate from a site within the cell nucleus, i.e. when the Gd-containing NCT agent accumulates in the cell nuclei. Studies in the literature demonstrate that GdNCT can be used to kill tumor cells,(15)(16) but do not differentiate the relative efficacies of the xcex3 rays and Auger electrons. It is often assumed that gadolinium diethylene triamine pentaacetic acid (Gd-DTPA, a gadolinium-containing compound that is used as an MRI contrast agent), does not penetrate the plasma membrane, but no experiment has been performed to tackle the issue of Gd-DTPA penetration (or other gadolinium-containing compounds) into tumor cells.
In the Examples that follow, human glioblastoma cells were exposed to the gadolinium-containing compound Gd-DTPA and then observed to determine whether the gadolinium accumulated intracellularly and/or intranuclearly. Three independent techniques were used to show cellular uptake of the compounds by cultures of tumor cells. One of these methods (inductively-coupled plasma mass spectrometry, ICP-MS) is a bulk analysis method, the other two methods (x-ray spectromicroscopy and Time of Flight Secondary Ion Mass Spectrometry, TOF-SIMS) are microchemical surface analytical methods. Spectromicroscopy is a well-established technique in materials science, but is only rarely used for the microchemical analysis of physiological and trace elements in biological specimens.(17)(18) As described herein, synchrotron spectromicroscopy was used to observe directly the intracellular distribution of gadolinium, via x-ray absorption spectroscopy at the Gd 3d edge. It was demonstrated that Gd-DTPA penetrates both the outer and nuclear membranes, and additionally shows preferential accumulation in the cell nuclei. This result has been confirmed by TOF-SIMS analysis.
Thus, the invention is directed to a method of selectively inhibiting the growth of neoplastic cells. The method comprises first contacting neoplastic cells (in vitro or in vivo) with a Gd-containing compound for a time sufficient to allow the Gd-containing compound to be internalized within nuclei of the neoplastic cells. The neoplastic cells are then exposed to a neutron flux of thermal and/or epithermal neutrons sufficient to induce a neutron capture reaction in Gd atoms within the Gd-containing compound internalized by the neoplastic cells. Growth of the neoplastic cells is thus inhibited due to cellular damage caused by Auger electrons and gamma radiation emitted by Gd atoms that underwent the neutron capture reaction.
The preferred embodiment of the invention is a method of treating glioma in mammalian subjects, especially humans, using the above-described GdNCT reaction. In the preferred embodiment, an amount of a non-toxic, Gd-containing compound (the amount being sufficient to allow the compound to be internalized within nuclei of glioma cells within the subject) is administered to a subject suffering from glioma. The subject is then exposed to a thermal and/or epithermal neutron flux sufficient to induce a neutron capture reaction in Gd atoms within the Gd-containing compound internalized by the glioma cells. Growth of the glioma cells is thereby inhibited (or the cells are killed outright) due to cellular damage caused by the Auger electrons and gamma radiation emitted by Gd atoms that underwent the neutron capture reaction.